1. Field of the Disclosure
This invention pertains to a method for preparing a relief printing plate from a printing plate precursor, and particularly to a method for preparing the relief printing plate from the printing plate precursor that is mounted on a support surface for treatment.
2. Description of Related Art
In one process for preparing a relief printing plate from a photosensitive element, the photosensitive element or printing plate precursor is often mounted around the circumference of a rotatable drum and heated to soften unpolymerized portions of the element for removal and create a relief surface of the printing plate. During thermal processing, the drum is rotated and the heated precursor is placed in contact with a development medium, such as an absorbent material, to remove the unpolymerized portions. It is desirable for thermal processing systems to accommodate printing plate precursors of different sizes, and in particular to mount printing plate precursors having different lengths around a circumferential surface of the drum. It is also desirable for the printing plate precursor to be securely mounted to the drum since during thermal processing the printing plate precursor needs to withstand the rigors of one or more cycles of heating and separating the development medium from the precursor. Defects can be induced in the resulting relief printing plate if during thermal processing the still warm printing plate precursor lifts, sags, or bends from the drum surface. The printing precursor can lift off the drum or sag and separate from the drum surface as the medium separates from the printing plate precursor, or as the precursor rotates on the drum. Uncontrolled separation of the medium and the lifting or sagging of the precursor while still hot bends the precursor and induces strains in the structure of the printing plate precursor which creates a defect, called waves, in the resulting relief printing plate. It is also desirable to keep the printing plate precursor firmly against the drum surface, particularly if the drum surface is cooled, to prevent or minimize heating of a back side of the precursor and to keep a support on the backside below its glass transition temperature. If the support of the precursor is heated to or above its glass transition temperature during thermal processing, the support can distort and result in deformations in the final processed relief printing plate. Further, the printing plate precursor can be securely mounted in contact with the drum surface by having the precursor held tautly or in tension to the drum. But the precursor should be properly tensioned during thermal processing since the precursor can change in one or more dimensions when heated. In some instances, the precursor can expand in length when it is heated and adjustments are needed to keep the precursor taut to the drum surface. The thermal development processor is capable of adjusting so as to maintain the desired tension on the precursor throughout thermal processing and properly remove of the unpolymerized portions from the relief surface as it is generated. If the printing plate precursor is not properly tensioned around the drum, and there is too little tension, the precursor may pull away from the drum and the unpolymerized portions may not be thoroughly removed. If the tension on the precursor is too high, the precursor can permanently distort or end portions of the precursor that attach to the drum can be damaged. Additionally, if the precursor is not appreciably held in contact with the drum, the precursor can rub against other parts of the processor and damage the relatively soft printing surface.
In some instances, the defects associated with lifting, sagging, and bending of the printing plate precursor can be exacerbated if only one end, i.e., a leading end, of the printing plate precursor is held to the drum, and the end opposite the leading end, i.e., trailing end of the precursor is not securely held to the drum. As disclosed by Peterson et al. in U.S. Pat. No. 5,279,697 the printing form precursor is secured to a preheating drum with a clamp that is flush mounted transversely on an outer surface of the drum. As disclosed by Odle et al. in U.S. Pat. No. 8,459,182 the printing plate precursor is mounted on the drum with both the leading end and the trailing end of the precursor securely held to the drum so that the precursor is properly tensioned and held in contact with the drum during thermal development. The thermal processor includes a first set of pins that are transversely coupled to the drum, a movable member that extends over a portion of the drum surface, and a second set of pins that are disposed transversely on the movable member. The leading end of the precursor is securely held by the first set of pins and the trailing end of the precursor is securely held by the second set of pins.
A problem arises with securely holding one or both ends of the printing plate precursor, whether gripped by clamps or by pins, since printable images cannot be created within the area or areas that are gripped. The gripped edge portion at the end of the photopolymerizable layer of the precursor, i.e., grip zone, is covered by the clamp or interrupted by the pins and thereby limits the surface area of the photopolymerizable layer that can be utilized to form a relief surface suitable for printing. Some customers desire the ability to produce printing plates with the print image content, i.e., printing relief surface, closer to the edges of the precursor than the areas for gripping permit. Some print jobs may require that the printing plate have printing relief surface within the grip areas to the very end edge or edges of the precursor. The end or ends of the printing plate precursor that cannot contain relief image for printing is a yield loss of the printing plate precursor. In some cases, the yield loss from the imageable surface of the photopolymerizable layer captured in grip zone/s can be as much as 10%, but typically from 3.5 to 6%, of the imageable area of the precursor.
As a consequence, there remains a need to provide a method for preparing a relief printing plate which allows for the entire or substantially the entire printable surface area of the precursor to contain image content and form relief surface for printing, while still securely holding at least one end of the printing plate precursor, preferably both ends of the precursor, to the drum or support surface in a processor.